1. Technical Field
The present disclosure relates to image sensors, and more specifically to a multiple conversion gain complementary metal oxide semiconductor (CMOS) sensor.
2. Discussion of the Related Art
Conventionally, a CMOS image sensor comprises pixels arranged in an array of rows and columns. Each pixel comprises a photodiode used in reverse mode, having its junction capacitance discharged by a photocurrent according to a received light intensity. At the end of a period, called image acquisition or integration period, before and after which the pixel is reset by recharging of its photodiode, the photogenerated charges stored in the photodiode are transferred to a capacitive sense node of the pixel. The measurement of the illumination level received by the pixel is performed by measuring the sense node voltage.
The sense node capacitance conditions the pixel sensitivity to illumination level variations. The lower the sense node capacitance, the higher the charge-to-voltage conversion factor or gain of the pixel, in volts per electron, and conversely. Thus, a sense node of low capacitance enables to relatively easily discriminate low illumination variations, especially in low illumination conditions, but may however be unable to receive all the charges photogenerated in the photodiode in case of a strong illumination. Conversely, a sense node of high capacitance enables to measure high illumination levels, but does not enable to easily discriminate low illumination level variations, especially in low illumination conditions.
Image sensors where the capacitance of the sense node of a pixel can be adjusted during the sensor operation have already been provided, which enables to adapt the charge-to-voltage conversion gain of the pixel, and thus its sensitivity and the extent of its dynamic range, according to the luminosity conditions.
It is however desired to improve certain aspects of existing sensors having an adjustable reading capacity.